The specific aims of this proposal are to elucidate the brain stem neurophysiological and neuropharmacological mechanisms controlling rhythmical jaw movements (RJMs), resembling mastication, in the guinea pig. We propose to characterize the pharmacological mechanisms controlling jaw opener (digastric) motoneuron and premotoneuron excitability during RJMs elicited by electrical stimulation of the cortex (or pyramidal tract), and during RJMs elicited by systemic administration of a dopamine agonist, apomorphine (APO). The project is divided into three parts. Parts one and two will focus on recording extracellularly from digastric motoneurons (DIG) or pre-motoneurons, respectively, during cortically induced RJMs while simultaneously applying to the cell, through micropipettes, small quantities of putative neurotransmitter (NT) agonists and antagonists. Finally, part three will investigate the neurophysiological and neuropharmacological mechanisms controlling these same cell populations during RJMs induced by APO administration. The long-term goal is to understand both the mechanisms underlying the central nervous system control of normal rhythmic jaw movements that occur during activities such as feeding and drinking, as well as the abnormal, involuntary rhythmic jaw movements occurring in disorders such as tardive dyskinesia and bruxism. Tardive dyskinesia is a movement disorder characterized by uncontrolled rhythmic jaw movements of oral-facial structures that result from long term administration of neuroleptic drugs. Bruxism, the grinding or clenching of teeth in sleep, is another uncontrolled jaw movement behavior that often leads to the myofacial pain-dysfunction syndrome. The etiology of the generation of these abnormal jaw movements in unknown, although brusixm is thought to be related to stress, and tardive dyskinesia to a disruption of dopamine activity in the basal ganglia. The results of the proposed experiments in the guinea pig will provide insights into the neurophysiological and neuropharmacological mechanisms underlying the production of involuntary rhythmic jaw movements in humans.